1. Technical Field of the Invention
The present invention relates to inrush current limiting switches and, more particularly, to a vehicular inrush current limiting switching circuit for a power supply, the switching circuit for enabling or disenabling the supply of electric power from a direct current power supply to vehicular electrical loads including a large capacity capacitor.
2. Related Art
With a hybrid vehicle, a secondary battery powered vehicle and a fuel cell powered vehicle, a direct current power supply, such as a secondary battery, a fuel cell or the like, supplies electric power via an inverter circuit to an alternating current motor for generating traction power. The inverter circuit converts direct current power of the direct current power supply into alternating current power for supply to the alternating current motor. The inverter circuit is switched on and off at a high frequency under PWM control for speed control of the motor. This causes variation of current to occur with the resultant higher harmonic current components, which are superposed on a power supply voltage as switching noise via wiring inductance while causing electromagnetic noise to occur. In order to absorb such switching noise, smoothing capacitors are connected in parallel to inputs of the inverter circuit in general practice. Such smoothing capacitors are also similarly applied to, in addition to the inverter circuit of the alternating current electric motor, a DC/DC converter circuit.
Further, for the purpose of checking, repairing and replacing the direct current power supply or electrical loads such as the inverter circuit or the like, a main power switch (first switch) is commonly connected between both of these component parts. In addition, in a case where a smoothing capacitor with large capacity is connected in parallel to the electrical loads, a large inrush current instantaneously flows from the direct current power supply to the smoothing capacitor when the main power switch is turned on. Similarly, the large inrush current may also flow, in another case where an electrostatic capacitance accompanied with the electrical loads is large. For limiting such inrush current, an inrush current limiting circuit, including an in rush limiting resistor and a subsidiary power switch (second switch) connected in series, is preferably connected to the main power switch in parallel thereto.
When the direct current power supply begins to supply electric power to the electrical loads and the parallel connected smoothing capacitor, the subsidiary power switch is initially turned on. This allows the smoothing capacitor to be charged at a slow speed via the inrush current limiting resistor. The main power switch is turned on at a stage where a terminal voltage of the smoothing capacitor is adequately raised, causing electric power to be directly supplied from the direct current power supply to the electrical loads.
Since the subsidiary power switch is turned on and off within short time intervals when commencing the supply of electric power to the electrical loads, the subsidiary power switch often includes a semiconductor-switching device whose switching timing can be easily controlled. The semiconductor-switching device has an advantage with a small size and low power consumption but encounters a shutoff failure causing a large leak current to flow regardless of an off state because of the semiconductor-switching device including a noncontact switch. Although such a leak current failure rarely occurs in the form of a sudden increase, it's often the case that the leak current increases with age resulting from progress in deterioration of the semiconductor-switching device.
In order to monitor an operating status of the vehicular electric system mentioned above and detecting a failure at an earlier stage to maintain performance of the vehicle, research and development work has been done to provide, for instance, an inverter-circuit failure detecting circuit disclosed in Japanese Unexamined Patent Application Publication No. 6-233402 and a capacitor failure detecting device disclosed in Japanese Unexamined Patent Application Publication No. 11-346476. Further, another development has been done to provide a detecting device, related to a shutoff failure of a subsidiary power switch, which is disclosed in Japanese Unexamined Patent Application Publication No. 2005-102471.
Such devices address issues wherein as the subsidiary power switch encounters the shutoff failure causing the flow of a large leak current regardless of an off state in nature, the circuit undergoes a defect such as overheating and, further, a risk of an electrical shock occurs during inspection of the circuit while causing the draining to occur in the direct current power supply.
However, with such devices, the inrush current limiting resistor, connected to the subsidiary switch in series, is used as a sensor to detect the leak current occurring in the shutoff failure of the subsidiary power switch. That is, measuring a voltage drop occurring across both terminals of the resistor on the basis of such leak current allows the monitoring to determine whether or not the subsidiary power switch encounters the shutoff failure.
However, the inrush current limiting resistor directly affects an inrush current limiting characteristic. Therefore, in designing a performance of detecting the leak current occurring in the shutoff failure of the subsidiary power switch, an issue arises with the occurrence of restrictions where the inrush current characteristic needs to be considered at the same time.
More particularly, these restrictions are listed as described below. With the subsidiary switch encountered with a failure, a leak current flows from the direct current power supply and passes through the electrical loads and the smoothing capacitor to the inrush current limiting circuit. Consequently, a resistance value of such a resistor determines a charging time constant of the smoothing capacitor, that is, an inrush current characteristic. For this reason, such a resistance value cannot be necessarily convenient for detecting the leak current.
This is because there is a requirement wherein the inrush current characteristic needs to be determined so as to rapidly charge the smoothing capacitor such that a vehicle is available to start to run within a limited time interval. Due to such a requirement, the resistance value needs to lie in a value less than a fixed value.
As a result, a voltage drop resulting from such resistor becomes less than a fixed value and, sometimes, the leak current becomes hard to be measured with a simplified circuit structure.